1. Field of the Invention
The invention pertains to the use of silane-crosslinkable coating formulations having good curing properties, which produce scratch resistant coatings.
2. Background Art
At present there is a great need for scratch-resistant coatings for a wide variety of applications. Particular mention might be made here of scratch-resistant topcoat materials for motor vehicles. In this context it is necessary in addition to differentiate between OEM coating materials and refinish coating materials. These coating materials differ primarily in their process temperature: while OEM coating materials are generally baked at 130-140° C.; refinish coating materials must be able to be processed at not more than 80° C.—even better would be 50° C., or even ambient temperature.
The great majority of the present commercial coatings for OEM and refinish are systems composed of isocyanate oligomers, in some cases the isocyanate groups of which are blocked, and hydroxy-functional polymers. These systems, however, still have a large number of various disadvantages.
For instance, on the one hand, the achievable scratch resistance is still not sufficient, so that, for example, in a car wash particles in the washing water cause noticeable scratching of the finish. Over time this permanently damages the gloss of the finish. Formulations would be desirable here that allow higher finish hardnesses to be achieved.
A further disadvantage of conventional automobile topcoat materials lies in the fact that they are solvent-based systems whose solids fraction is in some cases even below 60%. Because of the high molar masses of the uncrosslinked prepolymers and the correspondingly high viscosities and/or glass transition points thereof, it is virtually impossible to do without a solvent.
Finally, isocyanate-based systems possess the critical disadvantage that the isocyanate-containing components are not toxicologically unobjectionable and, moreover, have a strong sensitizing action. In the course of their use it is therefore necessary to take extensive precautionary measures in order to prevent inhalation of vapors or aerosols and to prevent skin contact. This is very inconvenient and expensive, particularly in the case of refinish applications. Replacing isocyanate-based coating materials by a more toxicologically unobjectionable system is desirable in any case.
The potential applications of scratch-resistant coatings are not restricted, however, to clearcoat materials for motor vehicles, but extend to many further areas: particularly for the scratch-resistant finishing of plastics, especially for transparent plastics such as corresponding polymethacrylates or polycarbonates, there is a high demand for coatings possessing superior scratch resistance.
On account of these disadvantages of the conventional isocyanate-based coating materials presently available commercially there is a keen search for new coating systems which no longer have the abovementioned disadvantages. In the case of one very promising approach the starting-point compounds are organic oligomers or polymers which possess hydrolyzable silyl groups of the general formula (1).—Si(OR)3-xR′x  (1)where:    R=alkyl or acyl radical    R′=alkyl, cycloalkyl or aryl radical    x=0 or 1.
These silyl groups are able in the presence of water—e.g., from atmospheric humidity—to undergo hydrolysis, with the formation of Si—OH functions, and subsequently to undergo condensation, with the formation of Si—O—Si bridges, as a result of which the coating cures. The silyl groups are attached in terminal or lateral position on the otherwise organic main chain of the oligomer or polymer, with bonding being via a hydrolysis-stable Si—C bond.
Polymers or oligomers which are able to crosslink to three-dimensional networks by groups of the general formula (1) are also referred to below as prepolymers.
In recent years a variety of coatings have been developed on the basis of such prepolymers, said coatings being distinguished not only by high hardness but also, in particular, by outstanding chemical resistance and weathering stability.
The hydrolyzable silane groups of the corresponding prepolymers are generally trimethoxysilyl groups or alkyldimethoxysilyl groups (general formula 1: R=methyl, x=0 or 1). For the preparation of the pre-polymers provided with these silane units it is possible to take a variety of pathways.
Thus, inter alia, EP-A-44 049, EP-A-267 698, EP-A-549 643 and U.S. Pat. No. 4,043,953 describe coating formulations which comprise prepolymers which have pendent silane groups. These prepolymers are prepared by copolymerizing ethylenically unsaturated alkoxysilanes with other unsaturated compounds. Preferably silanes containing (meth)acrylic groups are copolymerized with other (meth)acrylates to give alkoxysilane-functional polymethacrylates. In such a reaction it is of course possible for further unsaturated compounds such as styrene, for example, to be copolymerized as well. Disadvantageous features of this process include the high molar masses which are obtained, meaning that the corresponding polymers can be handled only in solution form.
EP-A-1 123 951 describes coatings which in addition to the above-described alkoxysilane-functional polymethacrylates, and further coating constituents, also comprise silane-terminated prepolymers which have been prepared from a polyol or alcohol having at least 2 OH functions and from an isocyanate-functional alkoxysilane. The coating materials prepared in that patent, however, are not solvent-free.
EP-A-571 073 describes silane-crosslinking coatings wherein the silane-terminated prepolymers are obtained by reacting isocyanates having tertiary isocyanate groups and amino-functional silanes. One of the disadvantages here is the difficulty of obtaining the tertiary isocyanates.
All of these attempts at producing coatings of high hardness which are suitable for producing scratch-resistant coatings and which crosslink via condensation of alkoxysilyl groups additionally have, without exception, a further critical disadvantage. Thus the preparation of the silane-functional polymers or oligomers starts exclusively from vinylsilanes of the general formula (2) or else from silanes containing groups corresponding to the general formula (3) which possess a propyl spacer between a heteroatom and the silyl group.vinyl-Si(OR)3-xR′x  (2)—X—(CH2)3—Si(OR)3-xR′x  (3)where:    R=methyl radical,    R′=alkyl, cycloalkyl, aryl or alkylaryl radical,    X=oxygen, sulfur or a group of the formula NR″,    R″=hydrogen, alkyl, cycloalkyl, aryl, aminoalkyl or aspartate ester radical,    x=0 or 1.
The reactivity of the silane-functional prepolymers obtained in this case, however, is no more than moderate. In order to achieve a sufficient cure rate with these components even at moderate temperatures of not more than 80° C. it is vital to add heavy metal catalysts—generally organotin compounds. Even at relatively high baking temperatures of 130-150° C. it is often not possible to do without heavy metal catalysts.
The avoidance of heavy metal catalysts—or at least a marked reduction in the amount of catalyst to be employed—would on the one hand be desirable from toxicological standpoints; on the other hand, the catalyst may also lower the storage stability of the coating formulation and the resistance of the cured coating material. For the same reasons the catalysis of film curing by strong organic bases such as 1,4-diazabicyclo[5.4.0]undec-7-ene (DBU) is likewise disadvantageous.
Furthermore, using the moderately reactive silanes of the general formulae (2) or (3), only methoxy-crosslinking prepolymers can be prepared, in other words prepolymers which give off methanol as they cure. Ethoxy-crosslinking systems, which give off the less toxicologically objectionable ethanol as they cure, are not possible, since compounds of the general formulae (2) or (3) with R=ethyl lack sufficient reactivities even in the presence of high concentrations of catalyst.
WO 92/20463 proposes adding the curing catalyst not to the topcoat material, with the silane-functional prepolymers present therein, but instead to a basecoat material. In a two-coat system first of all the basecoat, containing catalyst, is applied, and is subsequently covered with the topcoat material. Both coating films are dried or cured jointly, and the catalyst diffuses from the basecoat material into the topcoat film. Although this does allow the problem of the moderate storage stability of a one-component topcoat solution to be solved, it is not possible in this way to forego heavy metal catalysts or to achieve a reduction in the amount of catalyst.
DE-A-21 55 258 describes silane-terminated prepolymers which possess crosslinkable end groups of the general formula (4)—Y—CO—NH—Q—NH—CO—NR″—CH2—Si(OR)3-zR′z  (4)where:    Q=alkylene, cycloalkylene, arylene or alkylarylene radical,    R=alkyl radical, preferably methyl or ethyl radical,    R′=alkyl, cycloalkyl, aryl or alkylaryl radical,    R″=hydrogen, alkyl, cycloalkyl, aryl or aminoalkyl radical,    Y=oxygen or a group of the formula NR″,    x=0 or 1.
These prepolymers are distinguished by very high reactivity on the part of the alkoxysilyl groups, so that these prepolymers cure in air even in the absence of heavy metal catalysts. Ethoxy-crosslinking pre-polymers containing such end groups are also described. The prepolymers described in DE-A-21 55 258, however, are suitable merely for producing elastic coatings, but not for producing scratch-resistant coatings. Thus in the case of these materials each crosslinkable silyl group is attached to the prepolymer either by way of two urea units or else by way of one urea unit and one urethane unit. Urea units and, albeit to a lesser extent, urethane units as well, however, have a capacity to form hydrogen bonds which increases the viscosity and also the glass transition point of the corresponding polymers. Consequently prepolymers having crosslinkable end groups of the general formula (4) either possess only a low crosslinkable alkoxysilyl group density or because of their high urea and urethane group density are vitreous solids and can be handled only in highly dilute solution. Accordingly all of the prepolymers described in DE-A-21 55 258, with end groups of the general formula (4), either possess only a very low fraction of alkoxysilyl groups, of less than 3% by weight, or are prepared and used only in high dilution, as a 30% strength toluenic solution. For producing highly crosslinked and hence scratch-resistant coatings from low-solvent coating formulations, these prepolymers are unsuitable.
Moreover, the compounds described in DE-A-21 55 258 with crosslinkable end groups of the general formula (4) have considerable stability problems. Although the reactivity of these alkoxysilyl groups is high it can be neither controlled nor modulated. Accordingly these compounds are storage-stable and handleable in air only with severe problems and also only in solutions containing alcohol and acid anhydride. Their further processing presents similar problems.